Survival Of the Fetus: A Barrier Is Reached

By GINA KOLATA

Published: April 18, 1989

ALTHOUGH doctors have made great progress in helping severely premature infants survive, they have been unable to overcome a seemingly impenetrable barrier: if a baby is born before 23 or 24 weeks of pregnancy, experts say, it simply cannot survive. And nothing that medical science can do will budge that boundary in the foreseeable future.

Experts say that the overriding problem for these infants, as compared to those born after the 40 weeks of a normal pregnancy, is that their lungs are too immature to function, even with the help of respirators.

The issue has become important in the abortion debate. The Supreme Court's 1973 decision that legalized abortion held that the states did not have the right to bar abortion until ''the point of viability,'' which it set at approximately 24 to 28 weeks of pregnancy.

But opponents of abortion have sought to push that date back earlier. They would prefer that abortions generally be prohibited from the moment of conception, but barring that they would like the courts to define viability as occurring weeks before the point set by the Court in its landmark decision, Roe v. Wade.

In a dissent from one abortion ruling, Justice Sandra Day O'Connor wrote that improvements in medical science are pushing the point of viability ''further back toward conception.'' But experts on both sides of the abortion debate agree that, while the point of viability may be somewhat earlier than the one cited in Roe v. Wade, medical science has reached a biological limit in its ability to save premature infants.

Now the issue is before the Supreme Court again in a case involving a Missouri law that required doctors to assess fetal viability when a woman seeking an abortion was 20 or more weeks pregnant. Although the law did not explicitly bar abortion of a viable fetus, it would presumably be governed by Roe v. Wade's assertion that the state's interest in the life of the fetus becomes ''compelling'' after the fetus is viable.

Whatever the date of viability might be, the vast majority of abortions are performed well before it. About 90 percent of the 1.6 million abortions in the United States each year occur in the first three months of pregancy, said Susan Tew, a spokeswoman for the Alan Guttmacher Institute in New York, and fewer than 1 percent occur at or after 21 weeks of pregnancy.

The Missouri law was struck down by a Federal appeals court on several grounds. On the viability issue, the court declared that tests were inaccurate, medically inappropriate and expensive. But opponents and proponents of legal abortion say the Supreme Court could use viability questions as a way to reconsider the 1973 decision.

''It is a relevant issue since Roe v. Wade essentially defines life as 24 weeks of pregnancy,'' said Dr. Mary Ellen Avery, a professor of pediatrics at the Harvard Medical School. Dr. Avery, who has collected national data on the fates of infants of very low birth weight, is considered an authority by the American Academy of Pediatrics.

''It is true that infants born at 23 weeks or even a little earlier can live,'' she said. But she and other experts say that 23 weeks, when only about 10 percent of babies in the best newborn intensive-care units can survive, is probably a lower limit of viability.

A brief filed in the Missouri case by 167 scientists and doctors, including 11 Nobel Prize winners, argues that ''the earliest point of viability has remained unchanged at approximately 24 weeks of gestation since 1973.'' Development of Crucial Organs

It adds: ''There is no reason to believe that a change is either imminent or inevitable. The reason that viability has not been pushed significantly back toward the point of conception is that critical organs, particularly lungs and kidneys, do not mature before that time.''

Advances in technology have improved survival rates for premature infants born after 23 to 24 weeks. According to a recent report by the Congressional Office of Technology Assessment, only 10 percent of babies weighing less than 2.2 pounds, born before about 28 weeks of pregnancy, survived in 1960. Now half of them live.

The report said the advances in saving these babies were most pronounced in those weighing 1.6 to 2.2 pounds, or 750 to 1,000 grams, which corresponds approximately to 25 to 28 weeks of gestation. Two-thirds of these babies survive today whereas almost all died in 1960.

Dr. Avery said that in her own hospital, Brigham and Women's Hospital in Boston, about half of all babies born after 24 weeks of pregnancy survive and one-tenth of those born after 23 weeks survive. But, she added, ''The odds of survival become infinitesimal before 23 weeks.'' Lower Limit of Viability

Dr. Watson A. Bowes Jr., a professor of obstetrics and gynecology at the University of North Carolina in Chapel Hill, who says that he takes ''a strong pro-life position'' and who advises the National Right to Life Committee, said he views 23 weeks as about the limit of fetal viability. He has seen babies born at 23 weeks who survived but has not seen younger survivors.

''In our own hospital, the official lower limit of viability, when we will not do pregnancy terminations, is 23 weeks,'' he said. ''That is sufficiently well documented that we've really taken it as our limit.''

Dr. Jack Willke, a general practitioner who is president of the National Right to Life Committee, said he had seen newspaper accounts of babies who were born at 20 weeks of pregnancy and survived. He said hospital or medical personnel present at the births confirmed the reports.

''There have been no survivors that anyone knows of below 20 weeks,'' he added, but ''it could be that next week we'll save an 18- or 19-weeker.''

But neonatologists, pediatricians and many scientists say that have yet to see a 20-week-old fetus that survived. They say they suspect that the length of pregnancy in the cases Dr. Willke cites may have been miscalculated and that the fetuses he referred to may have been closer to 24 weeks.

It can be very diificult to determine the length of a pregnancy accurately. Many women do not know exactly when they became pregnant. And even the best methods of estimating a woman's stage of pregnancy are not entirely accurate, particularly late in pregnancy. Methods of Dating Pregnancy

The usual way of dating a pregnancy is by counting from a woman's last menstrual period. At best, this method will be accurate within a week for only 80 percent of women, said Dr. Sheldon B. Korones, director of the Newborn Center at the University of Tennesee, and one of the 167 scientists who signed the brief.

Dr. Korones said that the best way to date a pregnancy is with ultrasound pictures before 20 weeks of pregnancy. But even dates obtained from ultrasound early in pregnancy can err by a week in either direction.

The primary reason that babies younger than 23 weeks almost never survive is because their lungs are unable to breathe. During the first 23 to 24 weeks of fetal life, the lungs and capillaries that carry blood to the lungs are very immature, Dr. Korones said.

Before 23 or 24 weeks, capillaries have not yet moved close enough to the air sacs of the lung to carry gases to and from the lungs, and the lining of the air sacs is too thick to allow gas exchange. Moreover, the lungs are too stiff to expand and contract effectively.

''What it boils down to is that the blood vessels and the airway have not come close enough together nor have they developed sufficient maturity to function,'' Dr. Korones said. ''Time must pass. Maturity doesn't develop instantly. The way we've got it pegged now is 23 or 24 weeks. We're going to get better at saving babies that are that small. More will survive. But I don't think we'll be able to do much about babies that are much younger.'' Panel's Conclusion on Threshold

A committee of experts appointed by the New York State Task Force on Life and the Law to determine the age at which fetuses can survive outside the womb concluded last year that ''23-24 weeks is the threshold of fetal survival.'' Technical advances in the foreseeable future will increase the survival of infants above that threshold, the group said, ''but will not lower the threshold for fetal extrauterine survival.''

Babies born at 23 or 24 weeks also have very immature kidneys that cannot properly regulate urine. ''The ability to excrete waste products is extremely limited,'' said Dr. Arthur E. Kopelman, head of neonatology at the East Carolina School of Medicine, who has done research on the smallest premature infants.

Citing still another difficulty, Dr. Bowes said: ''Our microtechnology is at its limit. You have to feed these babies. Their veins are tiny. Their vessels are tiny. Just to get the equipment in, we've reached an impasse.''

Some researchers have suggested that it might one day be possible to devise an artificial placenta to take the place of a baby's immature lungs and kidneys if it is born too early. It would function like a heart-lung machine, recirculating the baby's blood, cleansing it and adding oxygen.

''In theory, you can imagine how you would do that, but in fact problems such as overwhelming and fatal infections would probably preclude it,'' said Dr. Kopelman. ''There are also problems with hemorrhaging. You have to anticoagulate blood so that it doesn't clot as it goes through the tubing. Tiny babies have a tremendous propensity to hemorrhage into their brains anyway,'' he said, and if they were put on an artificial placenta machine ''I think they would suffer tremendous brain hemorrhages.''

Dr. Leo Stern, professor and chairman of the pediatrics department at Brown University, said that to be nearly certain that a fetus will not be viable, abortions would have to be done before the 20th week of pregnancy.